1. Field of the Invention
The present invention generally relates to transistors and diodes; and more particularly, to a transistor and a diode which are constructed to have narrow trenches in which a crystal semiconductor is epitaxially grown.
2. Description of the Related Art
FIG. 20 is a cross-sectional view illustrating a conventional transistor 102.
The transistor 102, or a trench type power MOSFET, comprises a semiconductor substrate 111 in which N+-type impurity is highly doped in single crystal silicon and a drain layer 112 having an epitaxial layer of N−-type silicon, which is epitaxially grown on top of the semiconductor substrate 111.
A processing substrate 110 comprises the semiconductor substrate 111 and the drain layer 112, and is subjected to the process of fabricating a semiconductor device, such that a P-type body layer 113 is formed on an inner peripheral surface of the drain layer 112 on its surface side, and a plurality of P+-type ohmic regions 116 and N+-type source regions 130 are formed in the surface of the body layer 113.
Between the source regions 130, the surface of the semiconductor substrate 110 is etched in a straight line with predetermined width to form a narrow trench 120.
A gate insulating film 124 is formed on an inner surface of the narrow trench 120. The narrow trench 120 is filled with polysilicon without contacting the semiconductor substrate 110 due to the presence of the gate insulating film 124 therebetween. The polysilicon forms a gate electrode plug 127.
The gate electrode plugs 127 in each narrow trench 120 are connected to another via a gate electrode film (not shown) having a thin metal film.
Formed on the surfaces of the source regions 130 and the ohmic regions 116 is a source electrode film 137 having a thin metal film. Formed on the narrow trench 120 is an interlayer insulation film 131, which electrically insulates the source electrode film 137 from the gate electrode plug 127.
A drain electrode film 139 is formed on the backside of the processing substrate 110 (i.e., on the surface of the semiconductor substrate 111).
Suppose that a positive voltage equal to or greater than a threshold voltage is applied to the gate electrode film with the source electrode film 137 being made equal to a ground potential and a positive voltage being applied to the drain electrode film 139. In this case, formed at the interface between the gate insulating film 124 and the body layer 113 is an N-type inverted layer, which connects between the source region 130 and the drain layer 112 and through which current flows from the drain layer 112 toward the source region 130. This is the conduction state of the transistor 102. Since the transistor 102 has no JFET region that is usually provided for a power MOSFET having no narrow trench 120, the transistor 102 has a lower conduction resistance than a typical power MOSFET.
When the conduction state changes to a state where the potential of the gate electrode film is equal to that of the source electrode film 137, the inverted layer disappears causing no more current to flow.
In this state, the PN junction between the body layer 113 and the drain layer 112 is reverse biased; and the avalanche breakdown voltage of the P-N junction is equal to the breakdown voltage of the transistor 102.
In general, the avalanche breakdown voltage of a P-N junction depends on the shape of a depletion layer when reverse biased. Since the aforementioned transistor 102 has an electric field of nonuniform strength in the depletion layer that extends in the drain layer 112, the avalanche breakdown voltage depends on the electric field of the greatest strength; thereby, reducing the breakdown voltage.
In order to address this problem, there has been suggested a semiconductor device 103 having a structure as shown in FIG. 21, in which such an attempt has been made as to form a buried layer 122 having a different conductivity type from the drain layer 112, beneath the narrow trench 120 so as to decrease the electric field strength in the depletion layer that extends in the drain layer 112.
The buried layer 122 is formed by once digging deep the narrow trench 120 and then growing a filling material on the bottom and sidewalls in the narrow trench 120, where a single crystal or a poly-crystal semiconductor can be used as the filling material.
However, there is a drawback in not being able to obtain the breakdown voltage as expected in the case where single crystal silicon, selected as the filling material, is used to grow a silicon epitaxial layer in the narrow trench 120 to form the buried layer 122 of the epitaxial layer.